SIMFEROPOL, Ukraine — Russian troops took over the strategic Crimean peninsula on Saturday without firing a shot. The newly installed government in Kiev was powerless to react, and despite calls by President Obama for Russia to pull back its forces, Western governments had few options to counter Russia’s military moves.

Russian President Vladimir Putin sought and quickly got his Parliament’s approval to use its military to protect Russia’s interests across Ukraine. But while sometimes-violent pro-Russian protests erupted Saturday in a number of Russian-speaking regions of eastern Ukraine, Moscow’s immediate focus appeared to be Crimea.

Ukraine’s acting president, Oleksandr Turchynov, made a late night announcement that he had ordered the country’s armed forces to be at full readiness because of the threat of “potential aggression.”

Speaking on Ukrainian TV, Turchynov said he had also stepped up security at nuclear-power plants, airports and other strategic infrastructure.

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Ignoring Obama’s warning Friday that “there will be costs” if Russia intervenes militarily, Putin raised the stakes in the conflict over Ukraine’s future, evoking memories of Cold War brinkmanship.

After Russia’s Parliament approved Putin’s motion unanimously, with no abstentions, U.S. officials held a high-level meeting at the White House to review Russia’s military moves in Ukraine. Officials said Obama spoke with Putin by telephone for 90 minutes and expressed his “deep concern” about “Russia’s clear violation of Ukrainian sovereignty and territorial integrity.”

Administration officials said Obama told Putin that the United States is calling on Russia “to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine.”

A statement from Russia said Putin emphasized to Obama the existence of “real threats” to the life and health of Russian citizens and compatriots who are in Ukrainian territory. The statement indicated Russia might send its troops not only to the Crimea but also to predominantly ethnic Russian regions of eastern Ukraine.

“Vladimir Putin emphasized that, in the case of a further spread in violence in eastern regions (of Ukraine) and Crimea, Russia maintains the right to protect its interests and the Russian-speaking population that lives there,” the Kremlin statement said.

Obama told Putin he would support sending international monitors to Ukraine to help protect ethnic Russians. He said the U.S. will suspend its participation in preparatory meetings for June’s Group of Eight summit in Sochi, Russia, the site of the recently concluded Winter Olympics, warning that Russia’s “continued violation of international law will lead to greater political and economic isolation.”

In a move seen as a warning not just to the rest of Ukraine but to Western Europe, the Russian energy giant Gazprom said that Ukraine owes it $1.6 billion in back payments on natural gas, and that if it isn’t paid quickly, the country would lose its now traditional discount. In the past, Russia has cut gas supplies to Ukraine and parts of Western Europe when Putin wants to make a point.

The payment demand and loss of the discount would accelerate Ukraine’s financial crisis. The country is almost broke and seeking emergency credit from the International Monetary Fund.

NATO announced a meeting for Sunday of the North Atlantic Council, the alliance’s political decision-making body, and a meeting of the NATO-Ukraine Commission. NATO Secretary-General Anders Fogh Rasmussen said the allies will “coordinate closely” on the situation in Ukraine, which he termed “grave.”

But while Ukraine is a NATO partner, a status Russia also holds, it is not a member of the alliance and therefore is not entitled to automatic defense in the event of an attack.

The U.N. Security Council met in an open, televised session for about 30 minutes after closed-door consultations, despite initial objections from Russia to an open session. The council took no action.

“Russia and the West find themselves on the brink of a confrontation far worse than in 2008 over Georgia,” Dmitri Trenin, director of Carnegie Moscow Center, said in a commentary posted on its website. In Georgia, Russian troops quickly routed the Georgian military after they tried to regain control over the separatist province of South Ossetia that has close ties to Russia.

The latest moves followed days of scripted, bloodless turmoil on the peninsula, the scene of centuries of wars and seen by Russia as a crown jewel of the Russian and Soviet empires. What began Thursday with the early-morning takeover of the regional Parliament building by mysterious troops continued Saturday as dozens of those soldiers — almost certainly Russian — moved into the streets around the parliamentary complex and seized control of regional airports, amid street protests by pro-Russian Crimeans calling for Moscow’s protection from the new government in Kiev.

That government came to power last week after months of protests against its ousted president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia, its longtime patron, instead of the European Union. Despite the calls for Russia’s help, there has been no sign of ethnic Russians facing attacks in Crimea or elsewhere in Ukraine.

Ukraine’s population of 46 million is divided in loyalties between Russia and Europe, with much of western Ukraine backing closer ties with the European Union while eastern and southern regions look to Russia for support. Crimea, a semiautonomous region that Russia gave to Ukraine in the 1950s, is mainly Russian-speaking.

In his address to Parliament, Putin said the “extraordinary situation in Ukraine” was putting at risk the lives of Russian citizens and military personnel stationed at the Crimean naval base that Moscow has maintained since the Soviet collapse.

Putin’s parliamentary motion loosely refers to the “territory of Ukraine” rather than specifically to Crimea, raising the possibility that Russia could use military force in other Russian-speaking areas in eastern and southern Ukraine, where many detest the new authorities in Kiev.

Pro-Russian protests were reported Saturday in the eastern cities of Kharkiv, Donetsk and Luhansk and the southern port of Odessa. In Kharkiv, 97 people were injured in clashes between pro-Russia demonstrators who flushed supporters of the new Ukrainian government out of the regional government building and hoisted the Russian flag on top of it, according to the Interfax news agency.

In Crimea, the new pro-Russian prime minister — who came to power after the gunmen swept into Parliament on Thursday — claimed control of the military and police and asked Putin for help in keeping peace. There was no visible presence of Ukrainian troops Saturday.

Crimea became part of Ukraine in 1954, when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality when both Ukraine and Russia were part of the Soviet Union. The Soviet breakup in 1991 meant Crimea landed in an independent Ukraine.

Swedish Foreign Minister Carl Bildt summed the situation up: “What’s happening in Crimea is a Russian takeover. There is no doubt about that,” he told Swedish Radio. “Russian military forces are involved and there has been a local takeover of power.”

Moscow has remained silent on claims that Russian troops are already in control of much of the peninsula, saying any troop movements are within agreed-upon rules governing the semiautonomous Ukrainian region.